Restriction endonuclease analysis of genomic DNA from various inbred and wild mouse strains have been carried out using the Southern blotting technique and a xenotropic reactive recombinant DNA probe. Except for a few distantly related mus species, the DNA from all mice tested contained many copies of xenotropic proviral DNA. Of these 18-50 copies in each strain at least 4 appear to be common to all inbred mice and the remainder have been used to identify characteristic patterns of each strain which can be used to define genetic relatedness between them. As previously reported our studies of internal organization have allowed us to define at least 7 related but distinct families of xenotropic endogenous sequences in inbred mice. Using these patterns we hope to analyze the DNA from various tumors to see if any particular fragments show polymorphism compared to standard liver DNA. Attempts to clone specific identifiable sequences will continue. To determine why some of these endogenous sequences are expressed as xenotropic virus, as MCF recombinant virus or not all in some mouse strains but not others, continues as the primary long range goal of this project.